This invention relates to the processing of metal powders, for example, by a combination of thermally based (e.g. laser, or electron beam) Additive Layer Manufacturing (ALM) and subsequent heat treatments that then form a superalloy and in particular gamma prime phase containing superalloys.
Superalloys are alloys strengthened not only by the nature of their matrix and chemistry but also by the presence of special strengthening phases, usually precipitates. For a fuller description of superalloys see “Superalloys: A Technical Guide” ASM International ISBN 0-87170-749-7 and “The Superalloys: Fundamentals and Applications” Cambridge University Press ISBN-10 0-521-85904-2. These are alloys that have been developed recently for use in rocket and jet engines.
A superalloy is generally defined as an alloy with excellent mechanical strength and creep resistance at high temperatures, good surface stability, and corrosion and oxidation resistance. Some of the most useful form secondary phase precipitates such as gamma prime and these gamma prime precipitates frequently include titanium and aluminium. These superalloys present numerous processing challenges are frequently represented on a diagram as shown at FIG. 1. The inventor has broadly observed that those superalloys to the left of the ‘weldability’ line e.g. Inconel 718 can be ALM processed without bulk heating to a high temperature, whereas the superalloys to the right of the ‘weldability’ line cannot.
It is well known that functional metal parts can be manufactured from a variety of pure metals and alloys using ALM. Historically a so-called “Liquid Phase Sintering” process was used to form mechanically hard parts such as moulds from proprietary multicomponent metal powder e.g. DirectMetal 20 and DirectSteel—products of EOS GmbH. Liquid Phase Sintering describes a process where one lower melting point component of the powder is melted by the laser, but the other higher melting point components remain solid. DirectMetal is described as “bronze-based matrix containing nickel” by its manufacturers with a remaining porosity of 8% and no heat treatment required or described. DirectSteel H20 is described as a “steel based multi-component metal powder” which when laser sintered formed a steel alloy of greater than 99.5% density which has 5-10% improved tensile strength and 10˜15% improved yield strength after a heat treatment.
More recently with better suited lasers a full melt of many homogeneous metal powders has become commonplace. The powder feedstock for such parts is chemically homogeneous to the resultant alloy and made by the melting of a chemically homogeneous bar stock or elements and is of the composition required for the finished part.
In the best prior art ALM process the powder is fully melted where desired by the selective application of an energy source (typically a laser or electron beam) and then solidifies in order to produce, layer by layer, a fully dense metal part corresponding to a sliced design file. As the powder is fully melted and many metals and alloys have a high coefficient of thermal expansion the as-built part typically has considerable internal stresses and to retain dimensional accuracy the part is restrained by a build plate or jigs and fixturings during the build and throughout a subsequent heat treatment to substantially remove these stresses prior to removal from the base plate, jigs or fixtures. Additionally many metals go through a phase change as they cool from liquid adding further stresses.
In some alloys however—particularly nickel-based superalloys—the internal stress is sufficient to cause cracking of the part either during the ALM process or during the subsequent stress relief heat treatment. For example, an important subset of nickel superalloys with gamma prime alloying elements show this behaviour and are known to be difficult to work by conventional processes and are frequently classified as ‘unweldable’. These alloys are also of significant commercial interest as they are widely used for very high temperature applications—such as in combustion components in engines and frequently can only be cast and are difficult or impossible to repair.
The prior art solution is to add heat to the area of the melting metal to minimise thermal mismatching of the solid metal already formed and solidifying. In the case of lower melt temperature metals e.g. titanium, this may be a practical pragmatic solution, however for the superalloys particularly nickel based superalloys of interest it is either practically or commercially disadvantageous to heat the part because of the high temperatures necessary but also the time for which that temperature needs to be applied and the controlled cooling to achieve a sufficient stress reduction and no cracking.
Bourell U.S. Pat. No. 5,296,062 discloses the use of “powder comprising particles of a first material coated with a second material, said second material having a lower softening temperature than said first material”.
Bampton in U.S. Pat. No. 5,745,834 describes various methods for “selective laser binding and transient liquid sintering of blended powders”. These include a method where 3% Boron (a known melt point depressant) is added to a top layer of Haynes 230 superalloy powder representing 15% of the total layer thickness and a pre-heat applied to a temperature just below the melt point of the layer with Boron added. A laser is then applied to selectively melt the layer with the melt temperature depressant Boron. This liquid metal wicks into the 85% layer thickness beneath it and the Boron diffuses out of the liquid phase into the solid powder to produce “a nearly fully dense segment of the component.” Bampton points out that it is difficult to process at such a high (pre-heat) temperature on conventional equipment and that there is a significant temperature gradient from the laser melted spot of typically in excess of 100 deg C. and the bulk thereby creating residual stresses.
Bampton also describes a process where polymer powders are blended with the metal and a laser layer process carried out. The binder is burnt out leaving a porous metal powder solid state sintered together. This porous sintered part is then densified by either encapsulation and then Hot Isostatic Pressing or a lower melting point liquid metal infiltration. Both processes have significant disadvantages. The HIP'ed object will have substantially shrunk—and this process is laborious. In the case of the liquid metal infusion, the solid part is not 100% of the desired alloy and therefore does not have the desired mechanical properties.
Bampton then describes a mix of three powder components including the desired parent metal, the same base metal with melt point depressant and a polymer binder where the layers are built “by localized laser melting of the polymer constituent of the powder which rapidly resolidifies to bind the metal particles of the powder with connecting necks or bridges.” The binder is then eliminated in a furnace creating a low strength part (generally) requiring temporary support from e.g. ceramic powder during a transient liquid sintering process.
Also known (WO/0211928) are all metal powders (no polymer) with melt temperature depressant additives e.g. Boron or Carbon included. And the additions may be at a small percentage of total powder but when used as a discrete powder it may local present at far higher percentages thereby initiating melting, wetting and bonding of the powder to form an object that is substantially fully dense.
Similarly US2004/0182201 describes a process where “graphite is also used in sintering iron based powder for the purpose of lowering the melting point of the composition to be sintered . . . ” “graphite powder is considerably effective to improve the wettability during melting or to reduce microcracks during solidification of high-density portions”.
In Hede, WO 02/092264 the then current (November 2002) method and powders for Selective Laser Sintering with metal powders and laser (and all known free form methods) is described as not capable of producing a fully dense material-5-30% porosity remaining; Infiltration with a low melting point material being required.
Hede describes trials with tools steels leading to “a martensitic layer of high hardness and internal stresses making it difficult (impossible) to smooth the layer deposited with a scraper before applying the next layer” and with “a major risk of fissuring”.
Hede then describes the use of iron and copper based precipitation hardening alloys that would “give a soft material directly after laser sintering . . . the desired hardness could then instead by achieved by precipitation hardening . . . after laser sintering.” Iron alloys and in particular a maraging steel and 17-4PH stainless steel is described. Whilst the maraging steel 18NiMAR250 was tested Hede then goes on to speculate more broadly including 17-4PH as an example material that will precipitate harden.
At this time (2011) 17-4PH stainless steel alloy is one of the most widely used metal powders in laser Selective Laser Sintering equipment and has been so for many years. The applicants use it in their business on a daily basis. As recently as 2006 it was widely described (e.g. by EOS GmbH) as ‘precipitation hardening’ but in fact it has been found by the applicants that 17-4PH alloy powder does not precipitate harden after processing in the commercially available EOS M270 machine and 17-4 powder is no longer marketed as a precipitation hardening powder material. Clearly we are in a new area where broad speculation cannot be relied upon as a good guide to materials performance in selective laser processing plus post-build heat treatments.
Tegal DE 10039143 at [003] describes the problem to solve as being high levels of porosity when metallic components are produced from conventional powder mixtures. He describes a density of approximately 90% of the theoretical density with a steel powder and in laser-sintered parts of bronze, a residual porosity of about 30% remains.
The disclosure describes laser sintering a powder material comprising a mixture of at least two powder elements and is characterized in that the powder mixture is formed by iron powder as the main component and by further powder alloying elements, which are present in an elementary, pre-alloyed or partly alloyed form and that a powder alloy results from these powder elements in the course of the laser sintering process.
Tegal further amplified this by saying the powder alloying components are converted during the laser sintering process within milliseconds into a powder alloy, of which the component consists. Any subsequent treatments are described as for homogenization, stress relief annealing, heat treatment, reduction in internal defects and improvement in the surface quality.
It should be noted that the state of the art has advanced considerably since this disclosure was filed in the year 2000 and with current generation equipment (that fully melts rather than sintering the metal powder) porosity on the scale described by Tegal is no longer a problem.